Two-dimensional radial-ejection ion traps have been described extensively in the literature (see, e.g., Schwartz et al., “A Two-Dimensional Quadrupole Ion Trap Mass Spectrometer”, J. Am. Soc. Mass Spectrometry, 13: 659-669 (2002)) and are widely used for mass spectrometric analysis of a variety of substances, including small molecules such as pharmaceutical agents and their metabolites, as well as large biomolecules such as peptides and proteins. Generally described, such traps consist of four elongated electrodes, each electrode having a hyperbolic-shaped surface, arranged in two electrode pairs aligned with and opposed across the trap centerline. At least one of the electrodes of an electrode pair is adapted with an aperture (slot) extending through the thickness of the electrode in order to permit ejected ions to travel through the aperture to an adjacently located detector. Ions are radially confined within the ion trap interior by applying opposite phases of a radio-frequency (RF) trapping voltage to the electrode pairs, and may be axially confined by applying appropriate DC offsets to end sections or lenses located axially outward of the electrodes or central sections thereof. To perform an analytical scan, a dipole resonant excitation voltage is applied across the electrodes of the apertured electrode pair (often referred to as the X-electrodes because they are aligned with the X-axis of a Cartesian coordinate system, which is oriented such that X and Y are the radial axes of the trap and Z is the longitudinal axis extending along the trap centerline) while the amplitude of the RF trapping voltage is ramped. This causes the trapped ions to come into resonance with the applied excitation voltage in order of their mass-to-charge ratios (m/z's). The resonantly excited ions develop unstable trajectories and are ejected from the trap through the aperture(s) of the X-electrodes to the detectors, which generate signals representative of the number of ejected ions. The detector signals are conveyed to a data and control system for processing and generation of a mass spectrum.
It has long been recognized that the presence of the aperture(s) in the X-electrodes causes distortion in the desired quadrupolar trapping field, in particular adding negative octopolar (where both X-electrodes are apertured) and other higher even-order field components. These field distortions have been found to have operationally significant effects when the ion trap is employed for analytical scanning, including but not limited to ion frequency shifting and degradation of mass accuracy. One way in which ion trap designers have attempted to compensate for aperture-caused field distortions and minimize the associated adverse effects is by outwardly displacing the apertured electrodes (the X-electrodes), such that the apertured electrodes are positioned at a slightly greater distance from the trap centerline relative to the un-apertured electrodes. This outward displacement helps cancel (or can invert) the field distortions caused by the apertures in the electrodes. A drawback to this approach (commonly referred to as “stretching” the trap) is that when RF voltages are applied to the electrodes in the normal manner (whereby electrodes of one electrode pair receive a voltage equal in amplitude and opposite in polarity to the electrodes of the other electrode pair), the resultant electric field is not balanced, causing the centerline of the device to exhibit a significant RF potential. When ions are then introduced into the trap interior along the centerline, for a given RF amplitude the acceptance of the ions can be significantly m/z-dependent, which is an undesired behavior. Furthermore, ions in a misbalanced field can effectively oscillate with different frequencies in the X- and Y-dimensions, which eliminates the possibility of conducting phase-locked resonance experiments in both dimensions. In addition, due to the octopolar field component, the oscillation frequency shifts associated with a changing ion trajectory amplitude are in opposite directions for ion motion in the X and Y-dimensions.
Various approaches to balancing the RF field in a radial-ejection two-dimensional ion trap have been proposed in the prior art, including altering the hyperbolic surface profiles of the apertured electrodes to reduce their radii of curvature relative to the non-apertured electrodes (see U.S. patent application Ser. No. 11/437,038 by Senko, entitled “System and Method for Implementing Balanced RF Fields in an Ion Trap Device”), and applying RF voltages of different amplitudes to the apertured and non-apertured electrodes (see U.S. patent application Ser. No. 11/437,087 by Schwartz, also entitled “System and Method for Implementing Balanced RF Fields in an Ion Trap Device”). However, the implementation of these approaches may be difficult in practice and may significantly increase the cost and/or complexity of manufacturing and operating the mass spectrometer instrument.
Accordingly, there remains a need in the mass spectrometry art for a two-dimensional radial ejection ion trap which minimizes or eliminates the adverse performance effects of field distortion arising from the presence of the ejection apertures while maintaining a balanced RF electric field.